In recent years, active research has been conducted on an inkjet printing method using active energy ray. This is such a method that a liquid ink is applied to a paper, plastic or the like and then cured or cross-linked by, for example, UV irradiation. In contrast to a conventional inkjet ink printing method in which the ink is dried by heating, this method has advantages such that it is a fast-drying method; it uses no solvent; and it allows printing on media that absorb no ink. Inkjet methods need no plate upon printing, can eject and apply ink only to desired image areas, and can directly form an image on a recording medium. Therefore, attention has been focused on the inkjet printing method that uses active energy ray and allows efficient use of ink.
An active energy ray-curable ink composition generally uses no solvent. Therefore, in many cases, a polymerizable compound contained in the ink composition is used as a dispersion medium to disperse a pigment, so that the ink composition is often higher in viscosity than conventional ink compositions containing a solvent, and it faces difficulty in keeping ink stability over time.
In Patent Literature 1, a pigment dispersion containing a basic pigment dispersing agent having a comb-shaped skeleton, is disclosed as a technique for increasing the viscosity stability over time of a pigment dispersion containing C.I. Pigment Yellow 150.
In Patent Literature 2, an active light-curable inkjet ink containing a dispersing agent is disclosed, the dispersing agent having both an acid value and an amine value, and the acid value being larger than the amine value. In Patent Literature 2, it is mentioned that the technique of Patent Literature 2 makes it possible to record a high-resolution images stably, with high reproducibility.
However, the inkjet ink inventions of Patent Literatures 1 and 2 are problematic in that they show poor viscosity stability when heated. In addition, they are problematic in that they are likely to adhere to or remain on an ink ejection surface, such as the nozzle plate of an inkjet head, and to cause curved flight or nozzle clogging, therefore.
In the active energy ray-curable ink composition, a polymerizable compound is generally contained. Due to radicals or the like generated in the composition, a polymerization reaction may proceed in the composition during storage, which may be a cause of an increase in viscosity or a cause of gelation, therefore.
An ink composition has been proposed to solve the problem, which is produced by adding a polymerization inhibitor to inhibit polymerization reaction. For example, in Patent Literature 3, an active energy ray-curable ink composition is disclosed as a technique for improving viscosity stability, the composition containing phenothiazines and hindered phenols as a polymerization inhibitor.
Disclosed in Patent Literature 4 is an ink composition for forming a clear layer, containing at least one component selected from the group consisting of a hindered amine compound, a nitrosamine compound and a quinone compound as a polymerization inhibitor, for the purpose of obtaining an ink composition for forming a clear layer, the composition being able to maintain excellent viscosity stability for a long period of storage and to produce a clear layer with no haze and yellowing.
However, the techniques of Patent Literatures 3 and 4 are problematic in that they cannot provide an ink which is excellent in long-term viscosity stability both under an oxygen-free atmosphere (e.g., an aluminum bag) and in the presence of oxygen (e.g., a bottle) and which is excellent in curability. Also, they have such a problem that there is an increase in viscosity at a flow path or inkjet head in use, resulting in poor ejection stability, failure in homogeneous printing, and poor reproducibility.